The present invention relates to a solid state image pickup device having a shutter function and by utilizing a static induction transistor as a solid state imaging element.
There has been widely used a solid state image pickup device comprising a charge coupled device or an MOS transistor. The present tendency is to replace these elements by a static induction transistor (hereinafter referred to as SIT element).
As an example FIG. 1 shows a circuit arrangement of a solid state image pickup device constructed by arranging in a matrix a plurality of isolated gate SIT elements. FIG. 2 shows a waveform chart for explaining the operation of the isolated gate SIT arranged in a matrix as shown in FIG. 1. Respective SIT elements 1-1.1, 1-1.2, . . . , 1-2.1, 1-2.2, . . . are of an n channel SIT of normal OFF type and form a picture cell. Each picture cell receives light inputs and generates video signal outputs which are read out by an XY address system. As shown in FIG. 1 respective SIT elements 1-1.1, 1-1.2, . . . , 1-2.1, 1-2.2, . . . have source electrodes which are connected to respective vertical scanning lines 2-1, 2-2, . . . , drain electrodes which are connected to a reference voltage source (in the shown embodiment, grounded voltage) and gate electrodes which are connected to respective horizontal scanning lines 3-1, 3-2, . . . . The horizontal scanning lines 3-1, 3-2, . . . are successively selected by signals .0.G.sub.1, .0.G.sub.2, . . . generated from a horizontal register (not shown). The vertical scanning lines 2-1, 2-2, . . . are successively selected and connected to a readout signal line 5 through respective line selection switches 4-1, 4-2, . . . which are made conductive by signal .0.S.sub.1, .0.S.sub.2 . . . generated from a vertical register (not shown) during selected period of one horizontal scanning line.
When the signal .0.S.sub.1 having a duration equal to one line scanning period shown in FIG. 2a is supplied to the line selection switch 4-1 of the vertical scanning line 2-1 from the vertical register, the line selection switch 4-1 becomes conductive and source and drain electrodes of respective SIT elements 1-1.1, 1-1.2, . . . corresponding to respective picture cells in the horizontal direction are biased to a certain voltage during one line scanning period. Under this condition, when the signal .0.G.sub.1 shown in FIG. 2c is supplied to the horizontal scanning line 3-1 from the horizontal register, the SIT element 1-1.1 is selected so that the current equivalent to the charges caused by light, stored in a gate capacitance C.sub.g of SIT element 1-1.1 and having an amount corresponding to an amount of incident light, flows through a load resister 7, the transistor switch 4-1, and the SIT element 1-1.1 from a power supply source 6. As a result of this the variable component of the voltage drop generated in the load resistor 7 is obtained at an output terminal 8 as an output voltage corresponding to light incident upon the SIT element 1-1.1. In this way, after output signals corresponding to light inputs for the vertical scanning line 2-1 are read out successively from respective SIT elements 1-1.1, 1-1.2, . . . the following vertical scanning line 2-2 is selected by the signal .0.S.sub.2 shown in FIG. 2b from the vertical register so that signals .0.G.sub.1, .0.G.sub.2 . . . generated from the horizontal register shown in FIGS. 2c and 2d drive respective SIT elements 1-2.1, 1-2.2 . . . , source and drain electrodes of which are biased by the vertical scanning line 2-2 in the same manner as the above, and thus outputs of the SIT elements are successively read out. Same operations are successively repreated thereby obtaining required image pickup output.
In the conventional solid state image pickup device described above the signal reading out and signal resetting can simultaneously be performed for respective SIT elements constituting the picture cell so that only the shutter speed of a certain period determined by a scanning period from the time at which the given SIT element 1-1.1 is selected to the time at which the same SIT element 1-1.1 is again selected can be obtained. Under the condition that for example the line selection switch 4-1 is conductive by the signal .0.S.sub.1 thereby supplying the voltage of the supply source 6 to the vertical scanning line 2-1, when the signal .0.G.sub.1 is supplied to the horizontal scanning line 3-1 a part of charges stored in the gate capacitance C.sub.g corresponding to an amount of incident light for other SIT element 1-2.1 which is not selected on the same horizontal scanning line 3-1 is discharged through the source electrode thereof. In the television image pickup SIT elements of about 500 lines are arranged so that a releasing amount of light charges caused by such discharge at lower line can not be ignored and thus this leads to uneven or blur picture.
As a circuit construction of the solid state image pickup device using SIT element different from the above conventional device the construction shown in FIG. 3 is also considered. This construction is the same as that shown in FIG. 1 except that the light receiving portion (gate) of the SIT element corresponding to each picture cell is made floating condition as shown in FIG. 3 and the drain electrode of SIT elements is connected to the horizontal scanning line, so that the timing of drive pulse is also the same as that shown in FIG. 2.
In the construction shown in FIG. 3 charges generated by light input are stored in the gate capacitance of the gate electrode of respective SIT elements and the charges of one SIT element selected by signals .0.G.sub.1, .0.G.sub.2, . . . from the horizontal register in the SIT elements selected by the signals .0.S.sub.1, .0.S.sub.2, . . . from the vertical register are read out as an image pickup output corresponding to light charges stored by the application of the signals .0.G.sub.1, .0.G.sub.2, . . . and delivered from the output terminal 8. In FIG. 3 the same elements as that shown in FIG. 1 are given like reference characters.
In the embodiment shown in FIG. 1 when the horizontal scanning signal .0.G.sub.n (n=1, 2, . . . ) is supplied the stored light charges flow from the gate electrode to the source electrode of the SIT element corresponding to the cell on the same horizontal scanning line 3-1, 3-2, . . . other than the selected picture cell, but in the embodiment shown in FIG. 3, as long as the source electrode of the SIT element is not selected by the line selection switch 4-1, 4-2, . . . the charges stored in the gate electrode of the SIT element do not flow so that the image pickup output having no light blur may be obtained which is more advantageous than that of the previous device.
On the other hand, however, respective SIT elements shown in FIG. 3 can not release the charges stored in the gate electrode and generated by light. For example, in the solid state image pickup device comprising n channel SIT elements the holes in the gate electrode can not be released. Such structure can not be subjected to practical use without adding further improvement.
In order to image-pickup a moving object clearly, generally, it has to photograph the object with a shutter speed corresponding to the moving speed of the object and thus the shutter speed must be set to an optical time between 1/1000 sec and few seconds. In a television camera having a conventional image pickup tube, the NTSC system obtains image pickup signals as an output while performing the interlacing scanning at the frequency of 60 Hz per one field scan so that the photoelectric conversion surface of the image pickup tube must be charged up every 1/60 sec and thus the shutter speed lower than 1/60 sec can not be obtained. In the conventional device, for example, use is made of a mechanical shutter in front of the image pickup tube in order to obtain the shutter speed larger than 1/60 sec.
In the solid state image pickup device, for example, the television camera comprising CCD's, the electronic shutter function can easily be obtained by making charge storage time due to photoelectric conversion function variable and then the shutter speed can be optionally set by the external electric signals. The present invention is based of the above points.